How we Understand Dark Matter and Dark Energy is Changing

New research findings from DESI, has possibly unveiled universe altering findings after changing the way we might interpret dark matter and dark energy

To begin, scientists believe that out of all  matter in the universe, dark matter makes up 85% of the total mass. Dark matter is invisible, as it emits no light or energy, it cannot be measured by our regular sensors. Visible matter is known as baryonic matter, i.e. particles such as electrons, protons and neutrons.  As we cannot see dark matter, how do we know it exists? Starting off in the early 1930s theorists proposed that there was missing matter in their calculations, which was not observed  by experimentalists. Newton’s laws and general relativity end up failing on the scale of gravities. Modifications are needed to correct this, which cosmologists believe are explained by dark matter. [1]

Dark energy is a form of energy that cannot be measured but too in a similar way is needed for the theoretical models to work. The combination of dark matter and dark energy make up 95% of the total mass-energy content of the universe.

Backed up by experiments in the 1990s dark energy was seen to be a constant force, filling up space in the universe, driving the universes accelerated expansion. The dark energy’s magnitude is assumed to be non time dependent[2]. Continuous experiments have backed this up and the result has become a cornerstone in cosmology. However recent findings from the Dark Energy Spectroscopic Instrument (DESI), have brought this into question. DESI has created the largest ever 3D map of the universe, which included millions of galaxies and quasars. This model has hinted that the dark energy strength may evolve and depend on time, which could really shake up the previous models that were the foundation for many parts of cosmology. This has been hinted at in the past but as said by Professor Seshadri Nadathur form the University of Portsmouth, “It’s not just the data that continue to show a preference for evolving dark energy, but that the evidence is stronger now than it was”[3].  A researcher at DESI, Jiayi Cu has aptly spoke about the findings, saying “ We’re in the business of letting the universe tell us how it works, and maybe the universe is revealing that it’s more complex than we previously believed”.

The main model that these findings would have major ramifications for is the Lambda Cold dark matter model, which contains a cosmological constant, Lambda, which is associated with dark energy. On the more scientific side, the DESI analysis shows how the dark energy ratio of pressure to energy density, w, which is fixed at -1 in the Lambda Cold dark matter model, may vary over time. In the analysis done of the DESI data, the w value was found to be about -1.4, 11 billion years ago, and is only -0.7 today. This can lead to significant complications for our Lambda Cold dark matter model. Desi’s result may also suggest that the expansion of the universe was properly first accelerated 7 billion years ago, then peaked 2 billion years ago, and has been slowing down ever since.[4]

This disregard for a cosmological constant may however be seen as a win for string theorists, as Harvard’s Cumrun Vafa and his colleagues proposed that string theory cannot be compatible with a cosmological constant.[4]

By Edward Heeney

[1] E. Howell, “What is dark matter?,” Space.com, Apr. 16, 2013. [Online]. Available: https://www.space.com/20930-dark-matter.html

[2] C. Smith, “What is dark energy?,” Popular Science, Feb. 27, 2024. [Online]. Available: https://www.popsci.com/science/what-is-dark-energy/

[3]University College London, “Dark energy may evolve over time, suggests largest 3D map of universe,” UCL News, Mar. 20, 2025. [Online]. Available: https://www.ucl.ac.uk/news/2025/mar/dark-energy-may-evolve-over-time-suggests-largest-3d-map-universe

[4] D. Ehrenstein, “The Standard Cosmology Model May Be Breaking,” Physics, vol. 18, p. 72, Mar. 28, 2025. [Online]. Available: https://physics.aps.org/articles/v18/72